As oil is pumped from the ground, water typically seeps into the formation and the downhole pressure decreases. In a maturing well, the water cut of the production increases. A number of different techniques are used to increase oil recovery. A common method reinjects water into the formation to increase downhole pressure and flush residual oil into the producing wells. The oil-containing water may then be lifted by sucker rods and/or pumped to the surface by electrically powered submersible pumps placed downhole.
The downhole environment is particularly inhospitable to oil production equipment. In addition to corrosion, scale formation is a problem. Flush water containing a large amount of dissolved solids can form tenacious scales on equipment if co-ions are also present. Equipment subject to scale may require frequent cleaning or replacement. Scale formed by calcium carbonate, calcium sulfate, barium sulfate, dirt, and the like is typically encountered.
Various compositions are available for inhibiting scale formation from these typical scales. However, in some wells, zinc and/or lead sulfide is another source of scale. For example, water produced from wells of the upper mid-continent of the United States and along the Mississippi-Louisiana Gulf Coast contain relatively high concentrations of soluble Zn and/or Pb ions, typically as [ZnCl.sub.4 ].sup.2- and/or [PbCl.sub.4 ].sup.2- anion complexes. Kharaka et al., Applied Geochemistry, Vol. 2, Nos. 5-6, pp. 543-61, 1987 reported central Mississippi brine that contained 160,000-320,000 ppm of dissolved solids including 48,000 ppm Ca, 45 ppm Zn, 70 ppm Pb, 465 ppm Fe and 210 ppm Mn. Zinc sulfide scale can limit or even halt oil production by depositing on the interior surfaces of water carrying pipes or by causing oil pumps to stick or seize.
The calcium carbonate inhibitors are not generally effective in controlling these sulfide scales. Other dispersants have been developed specifically for the control of zinc sulfide scale, but adversely affect the performance of the calcium carbonate inhibitors, or are themselves adversely affected by the presence of the calcium carbonate inhibitors. In other words, calcium carbonate inhibitors and zinc sulfide dispersants which work well alone, do not generally work well in the same system. In addition, many of these inhibitors are adversely affected by the presence of iron.
As far as the applicant is aware, no technology has been developed for effectively controlling calcium carbonate and zinc sulfide deposits in an oil producing well in the presence of iron and therefore a need remains.